1. Field of the Invention
This invention relates to a technique which is suitably adapted for production of a substrate for a liquid crystal device, and a liquid crystal device and projection type display device based on the use thereof This invention relates more particularly to a light shielding structure of the substrate for the liquid crystal device which is used as a pixel switching element of a thin film transistor (to be abbreviated as TFT hereinafter).
2. Description of Related Art
Conventionally, a liquid crystal device is put into practice where pixel electrodes have been arranged in the form of a matrix on a glass substrate, and TFTs made of an amorphous silicon film or a polysilicon film have been prepared in correspondence with each pixel electrode, and which is so constructed as to drive a liquid crystal by applying a voltage through the TFT to each pixel electrode.
Among such liquid crystal devices, one incorporating a polysilicon film of which it is possible to assemble peripheral driving circuits such as a shift register or the like on the same substrate through the same process, allows a high density integration of circuit elements and attracts general attention.
With the liquid crystal device incorporating TFTs, the top of a TFT for driving a pixel electrode (to be referred to as a pixel TFT hereinafter) is covered by a light shielding film such as a chromium film called a black matrix (or a black stripe), which is placed on the opposite substrate. This is to prevent the channel region of the TFT from being exposed to direct light which would otherwise cause a leakage current. However, a leakage current caused by exposure of the TFT to stray light may arise as a result of light reflected from a polarizer placed on the back surface of the liquid crystal device, not to mention the adverse effects due to incident light itself.
To minimize such leakage current due to reflective light, an invention is proposed in which the back surface of the TFT is also covered by a light shielding film (Japanese Patent Publication, No. Hei 3-52611). If the light shielding film is placed on the back surface of the TFT such that it exceeds in size the opening of the black matrix placed on the opposite substrate, incident light strike: directly on the light shielding film, and light reflected therefrom illuminates the channel region of the TFT, which may cause it to generate a leakage current. This is because, when a process necessary for the placement of a light shielding film on the back surface of the TFT is put into practice, precise alignment of a black matrix placed on the opposite matrix with a pixel region placed on the substrate for the liquid crystal device is difficult, and thus incident light through the opposite substrate directly impinges and is reflected on the part of light shielding film that exceeds in size the opening of the black matrix. As a result, the channel region of TFT is illuminated, causing the leakage current to flow. Particularly when alignment of the light shielding layer placed on the substrate for the liquid crystal device with the black matrix takes place with a large error, light reflected from the surface of light shielding film increases considerably, and, as the channel region is illumined by this reflective light, a leakage current from the TFT is increased, resulting in a degraded display as a result of flaws such as cross-talks or the like.